Welding construction for multilayered devices

ABSTRACT

A welding construction for multi-layered device includes a plurality of concentric superposed tubes tightly fitted within one another and secured to one another by welding at the end of their interfaces formed between adjacent superposed tubes. A space is left between such welds and their associated interface in order to prevent stress concentrations and avoid cracking. Lengths of tubing thus formed are then assembled end to end by welding opposed end portions to each other. Welding in two steps enables the checking after each step.

United States Patent Pierart 14 1 Aug. 1, 1972 [54] WELDING CONSTRUCTIONFOR 3,497,101 2/1970 Sagara et a1. ..220/3 MULTILAYERED DEVICES3,557,839 1/1971 Utoet a1 ..l38/143 [72] Inventor: grobert Pierart,Saint-Nazaire, Primary Examiner M y Nelson rance AssistantExaminer-David R. Matthews [73] Assignee: Babcock-Atlautique, Paris,France Attomey-Young & Thompson [22] Filed: Feb. 22, 1971 [57] ABSTRACT4 [21] Appl' 117524 A welding construction for multi-layered deviceincludes a plurality of concentric superposed tubes 521 US. 01...138/143, 138/142, 29/4711, tightly fitted within one another andSecured to one 29/471] another by welding at the end of their,interfaces 51 Int. Cl .Q. ..F16l 9/14 famed between adjacent superposedtubes- A space is [58] Field of Search 142 4 137/1 left between suchwelds and their associated interface in order to prevent stressconcentrations and avoid cracking. Lengths of tubing thus formed arethen assembled end to end by welding opposed end portions [56]References Cited to each other. Welding in two steps enables the UNITEDSTATES PATENTS king f r each step.

2,489,802 11/1949 Needham ..29/471.1 11 Claims, 5 Drawing Figures v\L/G, \C4

I F 1F X 3 1 IF. I I F Q2 Q2 [F IF all WELDING CONSTRUCTION FORMULTILAYERED DEVICES BACKGROUND OF THE INVENTION 1 Field of theInvention The present invention relates to the construction of amultilayered device comprising a thick-walled metal casing formed of aplurality of superposed layers as well as such a device.

2. Description of the Prior Art One such known method of constructionconsists in welding lengths of thick-walled tubing end to end, thelengths being formed by nesting a plurality of thin walled tubes withinone another. These tubes are held tight within one another by heatingand expanding each tube intended to surround an inner adjacent tube andallowing it to cool and shrink on the latter. The multilayered lengthsthus formed are machined at their ends on a lathe as if they werethick-walled ordinary tubing and are then joined end to end by welding.

In Such a method of construction the faces which are to be welded havecontinuous lines of separation which correspond to the interfacesbetween superposed layers. These lines constitute notches havingvirtually no width. However experience has shown that after machiningthe edges of the lengths of tubing these notches may have a widthexceeding lrnrn without being in themselves detrimental to the qualityof the device.

In the known methods of manufacture automatic welding with a Union Meltpowder flux is ordinarily used in order to insure a homogeneous andcontinuous joining between the opposed edges of adjacent lengths.

These known methods of construction are well adapted to devices whichwork at moderate temperatures and do not use high-tensile steels whichare susceptible to cracking, as has been found in a great number ofapparatuses thus constructed which have been functioning for many years.

With respect to devices which are to operate at high temperaturescausing high thermal stresses or which use high-tensile steel inconjunction with known welding methods, the multi-layered device couldcause serious disadvantages as will be set forth hereinbelow.

In known welding methods, the welding operation is effected by thesuccessive deposit from one side to the other of the space between theopposed edges of adjacent lengths. It is clear that during this weldingoperation two results must be achieved:

1. the connection between the superposed layers of each length, and

2. the joining of adjacent lengths of multilayered tubes.

The checking of the connection between adjacent layers can be carriedout only after the welding operation is finished; such a checkingoperation is also difficult to effectuate. Repairing the weld is a verydelicate operation and involves new risks of faults or cracks. Checkingby ultra-sonic methods is equally difficult.

Experience has shown that in the course of such welding the level andthe penetration of the different passes or deposits relative to theposition of the interfaces can not be determined with sufficientprecision.

There prevail in such welding three principal types of dispositions ofthe weld relative to the interfaces at the edges of the opposed lengths,in particular at the socalled notches:

In a first type, the metal deposited during a welding pass is more orless centered on a notch. A dendritic formation appears in the moltenmetal as it freezes solid, with the dendrites starting to form at thecold end face of the composite tubing, perpendicular to that face. Theinterface, which is a crack of virtually no width, tends to propagate asa real crack in the solid weld, between dendrites.

In a second type, the metal deposited during one welding pass is offcenter relatively to the notch, and has a corresponding narrow sectionopposite the notch, which fact increases the risk of a crack developingbetween dendrites.

In a third type, the notch is relatively wide and the molten metalpenetrates up to a point between adjacent layers of tubing. The metalwhich has frozen solid between adjacent tube faces forms cusps withthese faces and cracks tend to develop at the cusps.

SUMMARY OF THE INVENTION The object of the present invention is toovercome the aforesaid disadvantages by providing a practical solutionto the welding problem of multi-layered pressure devices in particularwhen these devices are of great dimensions and formed of a high strengthsteel.

One aspect of the invention consists in a method of welding constructionfor a device including at least two lengths of tubing extending end toend, each length of tubing being formed as a plurality of superposedtubes tightly fitting within one another and interfaces existingtherebetween, comprising the following consecutive steps:

a. welding the end portions of adjacent superposed tubes of each of saidlengths of tubing to each other; and

b. subsequently welding the lengths of tubing thus formed end to end.

Another aspect of the invention consists in a method of weldingconstruction for a plurality of superposed elements, interfaces beingformed between adjacent superposed elements, the steps comprisingwelding the ends of adjacent superposed elements to one another in thevicinity of their associated interfaces and effecting a cavity betweeneach such weld and its associated interface.

These methods enable the elimination of such notches and cusps at theedges of the lengths in contact with the weld which as shown above causefaults and cracks.

A further aspect of the invention consists in a device comprising atleast two lengths of tubing, each length of tubing including a pluralityof superposed tubes tightly fitting within one another, interfacesexisting between each pair of adjacent superposed tubes, a plurality ofgrooves disposed at the end portions of the lengths of tubing andgenerally along the interfaces, adjacent superposed tubes of each lengthbeing secured to one another by first welds provided in said grooves,and the tubes extending end to end and joined together by a second weldeffected therebetween and independent of said first welds. I

Further objects, features and advantages of the invention will befurther brought out in the description BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 shows two lengths of tubing placed end to end used for producinga multi-layer device according to the invention;

FIG. 2 shows the detail A for FIG. 1 on a larger scale;

FIGS. 3-5 show three successive steps in the welding of two superposedthin tubes of a length of such tubing.

DESCRIPTION OF A PREFERRED EMBODIMENT The device to be constructed isformed by welding two lengths of tubing, for example lengths B and C ofFIG. 1, end to .end. Each one of these lengths comprises four thintubes, designated respectively 8,, B B B and C C,, C C The tubes formingeach such length are nested tightly within one another by heatingaccompanied by expansion and contraction following cooling, in a mannerknown per se. The interface IF between such tubes are shown in thedetail of FIG. 2.

Once such a length of tubing is constructed, the end faces thereof aremachined for example'with a lathe. This machining comprises forming asubstantially V- shaped groove G rounded at its inner end and centeredabout its associated interface at the edges of the interfaces betweeneach adjacent pair of superposed tubes. One such groove G correspondingto the interface IF between the thin tubes B, and B is shown in FIG. 3.

A relatively narrow secondary groove R is formed at the inner end ofeach of the grooves G. The passage communicating between the groove Gand the secondary groove R is limited or constrained by the swaging ofthe metal by a pneumatic tool or the like in order to I obtain thedeformed position shown in solid lines in FIG. 4, the original conditionbeing shown in chaindotted lines in this figure.

The groove R thus deformed can then be filled with a powdery flux,though this is not indispensable. A welding fillet is then effected ineach one of the grooves G.

This welding fillet shown in FIG. 5 is effected flatwise. Such lengthsof tubing are placed on a turntable having a variable speed control, theaxis of the length being arranged perpendicular to the plane of theturntable.

The weld is centered about its associated interface the followingstepsare carried out:

1. Welding at the inner end of the groove G, effected in two steps orpasses P P, by means of a manually operated arc-welder with basicelectrodes which-have a high resistance to cracking;

2. Checking of this weld by magnetoscope in order to verify the absenceof cracking;

3. Union Melt filling of the groove by automatic welding with a UnionMelt flux to the outer edge of the tube, using aswelding material wireof suitable gradeappropriate to the grade of the tube plates material;

4. Ultra-sonic checking of all the welds between super-posed layers.

The narrow groove R which may be filled with a powdered flux, and whichis constricted at its opened or outer end by swaging remains free offilled metal and forms a gap between the welding fillet and theassociated interface IF. The notch E situated at the end of theinterface IF is therefore not in contact with the weld and does not riskcausing a crack therein. Thus the crack existing at the interface IF isprevented I from propagating into the welding fillet.

The welding of opposed lengths B, C end to end is carried out in amanner known per se by means of a main weld S (FIG. 2).

The method described hereinabove provide certains advantages over knownmethods. In particular, the welding of the superposed layers of tubesfor a length of tubing is formed independently of the welding ofadjacent lengths end to end and therefore can be carried out under thebest conditions of manufacture and control.

Further the welding between the thin tubes is carried out underfavorable conditions, with the end face of the composite tubing disposedflat and fully accessible, while in known methods the weld whichconnects the thin tubes together must be deposited in a relativelynarrow space between opposite end faces of successive lengths of tubing.7

The groove R and therefore the welds between the tubes or layers arealways centered about their respective interface IF. Basic electrodeshaving an elevated resilience and a high resistance to cracking can beused for the interface weld.

There is no longer a risk of cracking nor of starting a crack in theweld owing to the special shape of the groove G and the presence of theempty cavity at the closed or inner end of groove G. The main weld Sbetween the opposed lengths is also out of contact with the notch- E. g

The checking is extremely easy and accurate and possible repairs do notcreate any difiiculties. The main 7 weld between lengths ofmulti-la'yered tubes is only undertaken after the checking of the weldsbetween each superposed pair of tubes has been carried out.

When the device is operating for instance as a container of hot fluid,thermal stresses occur because the temperature gradient across the mainweld S is not the same as across the superposed metal layers B through Band C through C These thermal stresses include shear stresses at theinterfaces IF and resistance to such shear stresses is increased by themetal filling in grooves G.

The invention is of course not limited to the embodiment illustrated anddescribed herein but extends to all variations, alternatives andequivalents within the scope of the appended claims. Accordingly, thearrangement of a cavity forming the gap between the two layers ofsuperposed tubes and the weld maintaining these layers in position isapplicable in a general manner to multilayered welded construction ofany type.

It is also noted with respect of the terminology used in the descriptionand in the appended claims, the term cluding at least two lengths oftubing extending end to end, each length of tubing being formed as aplurality of superposed tubes tightly fitting within one another andinterfaces existing therebetween, comprising the following consecutivesteps:

a. welding the end portions of adjacent superposed tubes of each of saidlengths of tubing to each other; and

b. subsequently welding the lengths of tubing thus formed end to end.

2. A method according to claim 1, further comprising forming a cavitybetween interfaces and the welds joining their associated pairs ofadjacent superposed tubes.

3. A method according to claim 1, further comprising forming a firstplurality of grooves at said end portions and generally centered alongthe interface disposed between the adjacent pairs of superposed tubes.

4. A method according to claim 3, further comprising forming a secondplurality of narrow grooves at the inner or closed ends of the firstplurality grooves, the narrow grooves thereby providing cavities betweentheir associated interface and the welds joining their associatedadjacent superposed tubes.

5. A method according to claim 4, further comprising shaping the ends ofthe narrow grooves adjacent to their associated groove for constrainingthe passage therebetween and thereby preventing the flow of metal intothe narrow grooves during welding.

6. A method according to claim 4, further comprising filling each ofsaid narrow grooves with a powdered flux before welding its associatedpair of adjacent superposed tubes.

7. A method according to claim 3, further comprising effecting thewelding of adjacent superposed tubes in two stages: (a) arc welding withbasic electrodes having high resistance to cracking, and (b) filling therest of the groove by automatic welding with a flux.

8. A method according to claim 7, further comprising checking the firststage of the weld with a mag netoscope before carrying out the secondstage, and ultra-sonically checking the second stage of welding beforeassembling the lengths of tubing end to end.

9. A method according to claim 1, wherein the welding of the endportions of adjacent superposed tubes is effected by flat welding.

10. A method of welding construction for a plurality of superposedelements, interfaces being formed between adjacent superposed elements,the steps comprising; welding the ends of adjacent superposed elementsto one another in the vicinity of their associated interfaces andeffecting a cavity between each such weld and its associated interface.

11. A device comprising at least two lengths of tubing, each length oftubing including a plurality of superposed tubes tightly fitting withinone another, interfaces existing between each pair of adjacentsuperposed tubes, a plurality of grooves disposed at the end portions ofthe lengths of tubing and generally along the interfaces, adjacentsuperposed tubes of each length being secured to one another by firstwelds provided in said grooves, and the tubes extending end to endjoined together by a second weld effected therebetween and independentof said first welds.

1. A method of welding construction for a device including at least twolengths of tubing extending end to end, each length of tubing beingformed as a plurality of superposed tubes tightly fitting within oneanother and interfaces existing therebetween, comprising the followingconsecutive steps: a. welding the end portions of adjacent superposedtubes of each of said lengths of tubing to each other; and b.subsequently welding the lengths of tubing thus formed end to end.
 2. Amethod according to claim 1, further comprising forming a cavity betweeninterfaces and the welds joining their associated pairs of adjacentsuperposed tubes.
 3. A method according to claim 1, further comprisingforming a first plurality of grooves at said end portions and generallycentered along the interface disposed between the adjacent pairs ofsuperposed tubes.
 4. A method according to claim 3, further comprisingforming a second plurality of narrow grooves at the inner or closed endsof the first plurality grooves, the narrow grooves thereby providingcavities between their associated interface and the welds joining theirassociated adjacent superposed tubes.
 5. A method according to claim 4,further comprising shaping the ends of the narrow grooves adjacent totheir associated groove for constraining the passage therebetween andthereby preventing the flow of metal into the narrow grooves duringwelding.
 6. A method according to claim 4, further comprising fillingeach of said narrow grooves with a powdered flux before welding itsassociated pair of adjacent superposed tubes.
 7. A method according toclaim 3, further comprising effecting the welding of adjacent superposedtubes in two stages: (a) arc welding with basic electrodes having highresistance to cracking, and (b) filling the rest of the groove byautomatic welding with a flux.
 8. A method according to claim 7, furthercomprising checking the first stage of the weld with a magnetoscopebefore carrying out the second stage, and ultra-sonically checking thesecond stage of welding before assembling the lengths of tubing end toend.
 9. A method according to claim 1, wherein the welding of the endportions of adjacent superposed tubes is effected by flat welding.
 10. Amethod of welding construction for a plurality of superposed elements,intErfaces being formed between adjacent superposed elements, the stepscomprising; welding the ends of adjacent superposed elements to oneanother in the vicinity of their associated interfaces and effecting acavity between each such weld and its associated interface.
 11. A devicecomprising at least two lengths of tubing, each length of tubingincluding a plurality of superposed tubes tightly fitting within oneanother, interfaces existing between each pair of adjacent superposedtubes, a plurality of grooves disposed at the end portions of thelengths of tubing and generally along the interfaces, adjacentsuperposed tubes of each length being secured to one another by firstwelds provided in said grooves, and the tubes extending end to endjoined together by a second weld effected therebetween and independentof said first welds.